FIG. 7 is a longitudinal sectional view showing a conventional circumferential flow liquid pump disclosed in Japanese Patent Publication No. 7-3239, for example. FIG. 8 is an enlarged sectional view taken along line VII--VII of FIG. 7. FIG. 9 is an enlarged sectional view taken along line IX--IX of FIG. 8.
In the figures, 1 designates an assembly of a pump casing, the assembly being constituted with the pump casing main body 2 and the cover 3. Within the pump casing assembly 1, an impeller 4 having vane portion 5 at its outer circumferential edge is disposed, the impeller 4 being supported by a central shaft 6 so that it is rotatable about its central axis relative to the pump casing assembly 1.
The pump casing assembly 1, as shown in FIG. 8, defines a pump flow path 7 of an annular band shape extending along the outer circumferential edge of the impeller 4 and a suction port 8 and a discharge port 9 opening to the opposite end portions of the pump flow path 7, the pump casing assembly 1 also accommodating the vane portion 5 of the impeller 4 within the pump flow path 7. Describing the further details of the pump casing assembly 1, as shown in FIG. 9, an air vent passage 11 opening from the bottom portion 10 of the pump flow path 7 and extending radially with a step therebetween is provided in the vicinity of the impeller 4 of the inner circumferential portion of the pump flow path 7 of the cover 3, and a through hole 12 having a sufficiently larger cross sectional area as compared to that of the air vent passage 11 for communicating the air vent passage 11 to the exterior of the pump casing assembly.
The central shaft 6 of the impeller 4 is constructed as the central shaft of the electric motor 15 connected to the circumferential liquid pump and its opposite end portions are rotatably supported by a bearing 17 and a bearing 18. 19 is an end cover provided with a check valve 22 and the liquid outlet 23 and holding the bracket 24. The pump casing assembly 1 and the end cover 19 are connected together by a yoke 20 of the electric motor 15. The yoke 20 accommodates therein a rotor 16, defines a liquid chamber 21 between the pump casing assembly 1 and the end cover 19 for storing the liquid such as a liquid fuel discharged from the discharge port 9 and has assembled to its inner circumferential portion a permanent magnet 25 serving as a stator. The liquid chamber 21 is communicated with the liquid outlet 23 having the check valve 22 disposed to the end cover 19, and inserted within the bracket 24 is a power supplying brush 27 brought in a sliding contact with a commutator 26 of the rotor 16.
Then, the operation of the conventional circumferential flow liquid pump will be described.
Also, within the pump flow path 7, gas in the form of bubbles due to the evaporation of the fuel generates at the liquid contacting surface of the vane portion 5 and the impeller 4 and tends to flow out into the liquid chamber 21. If these bubbles of gas flow into the liquid chamber 21 and reaches the internal combustion engine, various troubles can happen. Therefor, the arrangement is such that the gas in the form of the bubbles is discharged out of the pump casing assembly 1 as much as possible by the air vent passage 11 open to the inner circumferential portion of the pump flow path 7 and the vicinity of the impeller 4 and the through hole 12.
This function will be described in more detail below. During the operation of the pump, the gas in the form of the bubbles of the fuel vapor, which is generated at the contacting surface between the liquid such as the fuel and the vane portion 5 of the impeller 4 within the pump flow path 7, is collected at the inner circumferential portion of the pump flow path 7 in the vicinity of the impeller 4 due to the centrifugal force and the difference in specific weight from the liquid fuel and flows together with the liquid through the pump flow path 7 clockwise as viewed in FIG. 8 or in the same direction as the direction of rotation of the impeller 4.
When the gas bubbles reach about the air vent passage 11 which opens at the inner circumferential portion of the pump flow path 7 in the vicinity of the impeller 4 with a step raised from the bottom portion 10 of the pump flow path 7 and which extends in the direction coinciding with the direction of whirling flow 13 generated by the impeller 4 within the pump flow path 7, then the gas collected in the vicinity of the impeller 4 due to the static pressure within the pump flow path 7 due to the pumping action and the dynamic pressure due to the whirling flow 13 generated by the impeller 4 within the pump flow path 7 is forced to be introduced into the air vent passage 11. The introduced gas is discharged out of the pump casing assembly 1 through the through hole 12 having a cross sectional area sufficiently larger than that of the air vent passage 11.
In the conventional circumferential liquid pump as above described, if the bubbles of the fuel vapor is generated within the pump flow path and accumulated within the pump flow path 7, the so-called vapor lock may generate, impeding the flow of the liquid fuel and significantly decreasing the pump discharge rate. In view of these problems, the conventional circumferential flow liquid pump is arranged to discharge the bubbles out of the pump casing assembly through the air vent passage 11 open to the inner circumferential portion of the pump flow path 7 and in the vicinity of the impeller 4 as well as the through hole 12.
With the above structure, the depth (H in FIG. 9) of the air vent passage 11 must be made small, and in order not to increase the flow path resistance against the gas flowing through the air vent passage 11, it is desirable that the length of the air vent passage 11 is as short as possible. However, since the cross section of the through hole 12 is circular and the cross section of the air vent passage 11 is flat, the side wall of the air vent passage 11 for communicating the pump flow path 7 and the through hole 12 together are inevitably long. Therefore, under a bad condition in which a lot of fuel vapor is generated, the gas in the form of bubbles of the fuel vapor may not sufficiently discharged outside of the pump casing assembly 1, leading to a fear that the generation of the vapor lock cannot completely be prevented.
This invention has been made to solve the above discussed problems and has as its object the provision of an improved circumferential flow liquid pump arranged such that the gas such as the fuel vapor bubbles generated within the pump flow path is ensured to be discharged from the pump flow path to the outside of the pump casing assembly and there is no fear that the vapor lock generates.
According to the present invention, the circumferential flow liquid pump comprises an impeller having a vane portion in an outer circumferential portion thereof, a pump casing assembly rotatably supporting the impeller and defining therein a pump flow path of an arcuate band-like shape extending along an outer circumferential portion of the impeller and an suction port and a discharge port open at the opposite end portions of the pump flow path, and an air vent hole defined in the pump casing assembly which opens at one end thereof in an inner circumferential portion of the pump flow path in the vicinity of the impeller and at a position radially inwardly spaced from the bottom portion of the pump flow path, opens at the other end thereof to the exterior of the pump casing assembly at a position radially inward of the opening at the one end and which has a cross-sectional configuration disposed within a region of a partial annular ring shape extending along the pump flow path and a sufficiently large cross-sectional area.
Also, the air vent hole may comprise a radial passage extending from the one end thereof in a radially inward direction and an axial passage connected at its one end to the other end of the radial passage and open at is the other end to the exterior of the pump casing assembly.
Also, the air vent hole may comprise a radial passage extending from the one end thereof in a radially inward direction and a plurality of axial passages each connected at its one end to the other end of the radial passage and open at its the other end to the exterior of the pump casing assembly and disposed within the region of a partial annular ring shape.
Further, the air vent hole m ay comprise an axial passage directly extending from the one end and opening at its the other end to the exterior of the pump casing assembly.